Conventional laser systems are typically employed for processing target structures such as electrically conductive semiconductor link structures in integrated circuits or memory devices such as EPROMs, DRAMs, and SRAMs. Link processing, which is presented herein only by way of example of selective material processing, may include total or partial removal, cutting, or vaporization of the link material. Examples of link processing laser systems include model Nos. 8000C and 9000 manufactured by Electro Scientific Industries, Inc., which is the assignee of the present invention. These systems utilize output wavelengths of 1.064 .mu.m, 1.047 .mu.m, and 0.532 .mu.m.
The physics and computer modeling for laser-based link processing are described in "Computer Simulation of Target Link Explosion in Programmable Redundancy for Silicon Memory," L. M. Scarfone and J. D. Chlipala, Journal of Materials Research, Vol. 1, No. 2, March/April 1986, pp. 368-81, and "Explosion of Poly-Silicide Links in Laser-Programmable Redundancy for VLSI Memory Repair," Chin-Yuan Lu, J. D. Chlipale, and L. M. Scarfone, IEEE Transactions on Electron Devices, Vol. 36, No. 6, pp. 1056-1061 (June 1989).
FIGS. 1A and 1B depict a conventional output energy distribution of a laser output or pulse 10 directed at an integrated circuit or memory link structure 12, which can be positioned between link terminators 14, and is typically covered by a protective layer 16 often the result of oxide passivation. Link structure 12 may be composed of one or more layers of a single material or a composite "sandwich" of several materials including those required for anti-reflective coating, binding, or other manufacturing purposes. For example, link structure 12 may include sublayers of titanium and tungsten to enhance adhesion between aluminum base link material and a silicon substrate 22 which may include oxide layers.
With reference to FIGS. 1A and 1B, Chlipala et al. suggest that a laser pulse 10 focused to a spot 18 of radius R (which is, for example, about 2 .mu.m) and applied across link structure 12 should have a suitable duration or pulse width and be of sufficient energy at a certain wavelength to cause a temperature distribution capable of cutting link structure 12. Since the spatial or critical dimensions of spot 18 are typically (but not always) larger than the width (which is, for example, about 1 .mu.m) of link structure 12, a portion of laser pulse 10 impinges on silicon substrate 22. Laser pulse 10 must, therefore, be tailored not to have energy sufficient to damage silicon substrate 22 or adjacent circuit structures 20 either by direct laser energy absorption, by residual pulse energy coupled into substrate 22 below link structure 12 after it is vaporized, or by thermal conduction.
Yields of memory devices, for example, have been dramatically improved by combining the use of 1.064 or 1.047 .mu.m laser output with the use of polysilicon and polycide link structures 12 to enable redundant memory cells. Even though it is a relatively poor electrical conductor, polysilicon-based material has been used to fabricate link structures 12 because it is more easily processed by laser systems generating a conventional laser output at a wavelength of 1.047 .mu.m or 1.064 .mu.m at energy levels that do not prohibitively damage silicon substrate 22. A laser output having well-controlled energy and power levels can generate across an entire polysilicon link structure 12 a desired temperature distribution that exceeds the melting temperature of polysilicon. Scarfone et al. attribute this advantage to the relatively large optical absorption depth of polysilicon at 1.064 .mu.m in combination with other favorable parameters such as the mechanical strength, the thermal conductivity, and the melting and vaporization temperatures of polysilicon, protective layer 16, substrate 22, and other materials involved.
The technology trend is, however, toward developing more complex, higher density memories having more layers and smaller link structures and memory cell dimensions. As the complexity and numbers of layers of memory devices has increased, the polysilicon-like material has typically become more deeply buried and more difficult to process using a laser. Accordingly, an expensive and time consuming process is typically employed to delicately etch away surface layers to expose the polysilicon-like structures to be processed. Another disadvantage of polysilicon-like materials is that their electrical resistance increases with smaller dimensions, and thereby restricts the operating speed of the memory cells.
Because metals have higher conductivity and are typically deposited as a top conductive layer of memory devices, manufacturers would like to switch the material of link structures 12 to metals such as, for example, aluminum, titanium, nickel, copper, tungsten, platinum, gold, metal-nitrides (e.g., titanium nitride), or other electrically conductive metal-like materials in new generations of high-density, high-speed memory chips, whose storage capacity would exceed 4 and even 16 megabits.
Unfortunately, metals and other electrically conductive materials have much shorter optical absorption depths and smaller absorption coefficients at 1.047 .mu.m or 1.064 .mu.m than the absorption depth and absorption coefficient of polysilicon-like structures, causing most of the 1.047 .mu.m or 1.064 .mu.m laser output energy to be reflected away. Consequently, the small amount of laser output energy absorbed heats only the topmost portion or surface of a high conductivity link structure 12 such that most of the underlying volume of link structure 12 remains at a lower temperature. Thus, it is very difficult to cleanly process a high conductivity link structure 12 with the same laser output energy and power levels used to process polysilicon-like structures.
Simply increasing the laser output power level has deleterious effects on silicon substrate 22 and adjacent circuit structures 20. On the other hand, increasing the laser output pulse width, while maintaining the output power level, to allow sufficient time for thermal conduction to redistribute the heat to the underlying volume of a high conductivity link structure 12 increases the cumulative laser energy of an output pulse, thereby increasing the risk of damage to substrate 22 and circuit structure 20. Thus, some practitioners have concluded that laser systems are no longer the proper tool for processing high conductivity links and have discussed using ion beams instead ("Focused Ion Beams," Jon Orloff, Scientific American, October 1991, pp. 96-101). However, ion beam technology is still largely experimental for such applications, is very expensive, is not an automated production process, and cannot easily be retrofit into existing laser-based link cutting systems.